1. Field of the Invention
The present invention relates generally to energy harvesting systems. More specifically, the present invention relates to a broad band energy harvesting system and related methods to harvest energy from the structure.
2. Description of the Related Art
Various types of platforms such as, for example, aircraft structural components, aircraft skins, or other related components, when in operation are subjected to various environmental conditions such as stress and strain, exposure to temperature extremes, and significant vibration energy. Due to the various environmental conditions such components can suffer material degradation over time.
Structural health monitoring helps promote realization of the full potential of such components. Remotely positioned sensors and/or nodes have been installed adjacent to such structures/components to monitor various parameters such as, for example, strain levels, stress, temperature, pressure, or vibration level to help manage physical inspection schedules, maintenance schedules, to help predict material failure, and generally monitor the “health” of such components.
Such sensors have been provided a dedicated power supply such as power obtained through conductors, e.g., wires, connected to the aircraft electrical system or through chemical batteries. Such wiring can, in some instances, undesirably result in increased weight and complexity of the component being monitored and/or the associated structure or component and are subject to damage or breakage requiring extensive repair costs and down time.
Depending upon available space, batteries can be inappropriate due to their size. Batteries can also have a limited service life and therefore, typically require periodic inspection and/or replacement, are often positioned in locations difficult to reach, and often require costly disassembly and reassembly of the sensor or component to perform service on the battery. Further, batteries may not be suitable due to environmental constraints, i.e., temperature changes often affect battery performance.
Other structural health monitoring systems include self-powered sensors attached to or embedded within the components to be monitored that can reduce dependence on batteries or any other external power source. The sensors can include an energy harvesting device either incorporated within the sensor or connected externally. Such sensors can be relatively small in size and can utilize, as a power source, energy obtained or otherwise transmitted through the component or structure being monitored. This type of sensor can typically consume very low amounts of power in the microwatt range.
The energy harvesting devices for these sensors can generate small electrical currents, for example, when the material is deflected, such as when the monitored component vibrates. To do so, such devices typically include one or more cantilevered beams weighted with a proof mass on the free end and connected to a base on the opposite end, which is connected to or in direct contact with the vibrating structure. Each cantilevered beam can form a resonator or generator. Resonators and resonant systems have a natural frequency. Such resonators and resonant systems respond to frequencies close to their natural frequency much more intensely than to other frequencies. Each beam is sized and/or weighted to vibrate at a preselected resonant frequency coinciding with the expected frequency of the vibration energy generated by the structure. Such devices are designed with a relatively high quality factor and corresponding narrow bandwidth in order to maximize harvesting energy from the structure at the expected vibration frequency. This quality factor is a measure of the “quality” of the resonator, which in a mechanical system indicates, the effect of mechanical resistance to resonance—a high-quality factor equates to low mechanical resistance.
Each beam also generally has a material attached which generates electrical currents when the beam, and thus the material, is deflected by the vibrations. Piezoelectric material is but one example of materials that perform this function. As each beam deflects at the preselected resonant frequency due to the vibrations generated by the structure, the piezoelectric material converts a fraction of the mechanical energy to electrical energy. An electric circuit including a storage device, such as a capacitor, is typically connected to the piezoelectric electric material to receive and store the generated electricity for use by the sensor or node.
Where the vibration energy is expected at more than one discrete frequency, multiple energy harvesting devices can be deployed with each tuned to resonate at a separate preselected discrete resonant frequency coinciding with each of the discrete vibration frequencies expected to be generated by the structure. In another design, each of the beams or group of beams in a single energy harvesting device can be separately sized and/or weighted to vibrate at a separate preselected discrete resonant frequency coinciding with each of the discrete vibration frequencies expected to be generated by the structure. As with the single resonant frequency-multiple resonator design described above, this multiple resonant frequency-multiple resonator design is configured so that each of the beams (resonators) are designed with a relatively high quality factor and correspondingly narrow bandwidth in order to maximize harvesting energy from the structure at the expected predetermined vibration frequencies.
Because power harvesting relies on energy being available in the vicinity of the energy harvesting device, the sensors positioned in the areas of environmental energy having a frequency different than the expected frequency often do not receive sufficient power to provide continuous sensing capability necessary to perform even sampled sensing using a small duty cycle. Also, the available energy distribution may change such that an area of the structure once having a high level of environmental energy at an expected frequency or frequencies is now subject only to a low-level of such energy at such frequency or frequencies, making power availability less reliable. In such situations, energy may nevertheless be available at one or multiple frequencies spaced over and/or shifting over different frequencies between a known frequency band. Further, an operator installing the energy harvesting device may not know anything more than a range of potential frequencies that the energy harvesting device will have access to until immediately prior to actually installing the device in or on the vibrating structure, i.e., selecting the location on the structure to position the energy harvesting device. As such, each individual energy harvesting device may need to be manually tuned according to its selected location, or groups of devices having different predetermined resonant frequencies may need to be manufactured separately for different portions of a structure expected to experience different but known vibration frequencies.
Recognized by the Applicants is that energy harvesting devices having one or more discrete resonant frequencies configured to have a relatively high quality factor, and thus, a correspondingly relatively low bandwidth, may be unreliable in the expected frequency or frequencies of the environmentally generated vibration energy of the structure if the expected frequency or frequencies of the vibration energy do not fairly precisely match the actual frequency or frequencies of the environmentally generated vibration energy. Correspondingly, also recognized by the Applicants is the need for a broad band energy harvesting system capable of harvesting energy over a relatively broad frequency band so that as the frequency of the environmentally generated energy shifts, the broad band energy harvesting system can continue harvesting energy, and thus, supply power to the associated sensor or sensors. Further recognized by the Applicants is the need for a single energy harvesting device or apparatus capable of working over a very broad range of frequencies that would be available in different locations on a structure.